The present invention relates to a method and apparatus for improving the performance of a separator. More particularly the invention relates to a method and apparatus for improving the performance of a separator used for separating crude oil and water in oil production well-streams.
The production well-stream from an oil well may contain a mixture of oil and water (and may also include gases, solids and other fluids). The water may arise naturally, or may be a result of water (e.g. seawater) pumped into the well to raise the pressure and force more oil out. Separation of the various constituents is an important part of the processing, which it is desirable to carry out as soon as possible before the crude oil is stored or transported for further processing. Although oil and water are immiscible liquids, separation is particularly problematic when the water is carried in the form of small droplets in a continuous oil phase. In some situations the oil and water form an emulsion of very small water droplets dispersed throughout the oil.
Most separation processes include a settling vessel in which oil and water phases separate under gravity, with the heavier water falling to form a water layer below a lighter oil layer.
The gravity separation process requires a sufficient residence time in the settling vessel for the water droplets to sink to the water layer. The effectiveness of the separation depends not only on the residence time but also on the nature of the oil and water mixture entering the vessel. In many situations, an emulsion layer forms at the interface between the oil and water layers. The emulsion layer (also known as a rag layer) tends to inhibit the separation and prevent water droplets sinking into the water layer. Furthermore, in these situations the emulsion layer tends to build up over time, further retarding the separation process and reducing the volume of oil and water layers in the settling vessel.
It is known to reduce the problem of a build-up of the emulsion layer by removing a portion of this layer through an outlet provided at an appropriate position on the settling vessel. This may be done periodically in a batch operation after the emulsion layer has been allowed to build up for a time. The emulsion removed may be taken away for treatment or disposal.
WO 92/19347 and WO 92/19349 describe processes in which a stream is taken from the emulsion layer in a separating vessel and fed into and oil-water hydrocyclone separator before being returned to the separating vessel inlet. A hydrocyclone may be effective in aiding separation of oil droplets from a water-continuous emulsion, but has a problem in that it is only effective over a limited range of oil/water proportions. Also hydrocyclones are not effective in separating water droplets in oil-continuous emulsions. Another problem with using a hydrocyclone is that although some degree of separation and coalesecence of droplets may be achieved, much of this is destroyed at the outlet where the shear forces tend to remix the liquid phases.
Even when there is little or no emulsion layer between the oil and water layers, the size of the settling vessel is limited by practical or economic constraints such that the fluids have insufficient residence time in the vessel for the smallest water droplets to drop out of the oil.
It is an aim of the present invention to provide a method and apparatus for improving separator performance so as to alleviate the aforementioned problems.